Question

In 1935, cane toads were first introduced to Australia from Hawaii as a biocontrol agent. The toads were meant to replace the use of toxic pesticides to reduce the native cane beetle population that was decimating sugar cane, a valuable Australian crop. Since their release, however, cane toads have rapidly multiplied: over 70 years, the population increased from 100 individuals to over 200 million! The toads have devastated local ecosystems, including spreading diseases to native wildlife, preying on native fauna, and out-competing native insectivores for food. Additionally, no evidence has been found that they have had any effect on the cane beetles that they were meant to control. What type of growth model does the cane toad population follow?

Answer 1

Exponential growth model

Step-by-step explanation:

Populations that experience exponential growth models show an increase in proportion to their size. The increase in the population size depends on the individual´s reproduction rate: a population that grows at constant exponential rate gains individuals faster as the population increases in size.

This model states that populations live in an environment with unlimited resource availability, so the density factor does not influence population growth. There is no density-dependence effect nor competition for resources. Natality and mortality rate do not depend on density. There is a constant growth rate per capita, and it is proportional to the population size.

Exponential growth is hardly seen in nature due to its characteristics.

Almost every environment shows a carrying capacity point. Carrying capacity is the capability of the environment to support a population.

Commonly environments have limited resources and space, and as the population grows, competition increases too. The per capita growth rate is dense-dependent. Eventually, the population reaches the maximum point of carrying capacity, delimited by available resources, such as food or space.

The cane toad is an invasive omnivore species, feeding on anything, going from other animal species to invertebrates and vegetables. In urban areas, they might even feed on dogs food or human food wastes. The species characterizes by its high adaptative capability to different environments, the significant reproductive rate -leaving abundant offspring-, and the toxins produced in the parotids. For these traits, it has become a severe invasive species. Their poison is dangerous and painful, causing irritation of the skin and mucose, and many times, causing blindness. Predators get in contact with this toxin and get poisoned to death. Many native predators have suffered a reduction in their population size due to the effects of the Bufo toad. Also, as they prey on many other species, they eventually affect the ecosystemic equilibrium, driving many native species to extinction.

According to the description of the species and the information provided, we can assume that the population is going through exponential growth. Resources seem to be abundant enough, their reproductive rate is very high, and they are good competitors to native species. In 70 years, its population increased from 100 individuals to 200 million and still growing. It does not seem to be any limiting factor (or not yet at least), nor space nor food or shelter affecting the development. Unless there is something that might limit it, the species will keep showing exponential growth.